The Waffle switch `block_structure.storage_backing_for_cache` was removed in https://github.com/openedx/edx-platform/pull/35185 -- this is just a lingering reference in a setting comment.
The Waffle switch `block_structure.storage_backing_for_cache` was removed in https://github.com/openedx/edx-platform/pull/35185 -- this is just a lingering reference in a setting comment.